The present invention refers to a process for the thermal treatment of steel strip and, more precisely, refers to the thermal treatment both of as cast steel strip using the so-called strip-casting technique, and of hot-rolled strip. The process is moreover suited for the treatment of any type of steel.
Normally, steel strip, either directly continuously cast or hot-rolled, is wound, when it is still at a high temperature, in coils which are left to cool down at room temperature. However, as is well known to those skilled in the field, the strips thus rolled do not possess characteristics suitable for a subsequent cold-rolling treatment, in particular as regards their microstructure, homogeneity of composition, and their mechanical characteristics. Consequently, it is necessary to bring the coils to a high temperature for a time sufficient for bringing about the necessary changes, with a treatment referred to as annealing.
Annealing may be either of the continuous type or of the discontinuous type. Continuous annealing is carried out in a furnace heated at a high temperature, through which the strip is made to pass at a certain speed. Continuous annealing permits a uniform quality of the treated strip and a limited treatment time, but entails large and costly plants.
In discontinuous annealing, the strip is wound into coils, which are then loaded into a furnace. In this case, the plant is simple, not particularly cumbersome, and relatively economical, but the process of treatment is very long, generally in the region of a few dozen hours, and the end quality of the product is uneven.
For the treatment of strip that is directly continuously cast or hot-rolled, the annealing method most widely used is the discontinuous one, which presents evident disadvantages in terms of waste of energy, time and resources, and the resulting quality is not uniform.
A possible solution to the problems referred to above may be that of transporting the coils from the winding stage to the annealing furnace without allowing them to cool down excessively.
In this connection, so far attention has been focused on the treatment of stainless steels, or in any case corrosion-resistant steels. For example, the published Japanese patent application No. 52-65126 describes a process for the thermal treatment of stainless steels (of the types SUS 410 and SUS 430), in which the stainless-steel coils are loaded still hot into the annealing fumace. Likewise, the European patent application No. 343 008 refers to the treatment of hot-rolled stainless-steel strip, or in any case corrosion-resistant strip, in which the strip is hot-rolled above the transformation temperature A3 and then cooled down at a rate of between 10 and 1 xc2x0 C./min, in order to prevent the presence of martensite. This is obtained by isolating the strip against excessive heat losses, at least in part enclosing it in a thermally insulated casing.
The experience acquired through long experiments carried out by the present applicant has revealed that the teaching that may be drawn from the known art does not appear satisfactory, in particular for strip of small thickness, for example less than 3 mm. Furthermore, the known art is declaredly applicable only to stainless steels, or in any case corrosion-resistant steels. In addition to these points, the applicant has identified a number of process parameters not taken into consideration by the known art, which appear essential in order to achieve high-quality results.
The purpose of the present invention is, therefore, to enable hot treatment of steels of any type, cast directly in continuous casting or hot-rolled, in particular to small thicknesses, to obtain in the treated strip an excellent uniformity of composition and microstructure, in particular the absence of martensite, and hence high and uniform mechanical properties, not inferior to those obtainable from traditional annealing processes.
Amongst the advantages of the present invention, which are obvious to those skilled in the field, we recall that of an important energy saving.
According to the present invention, the process for thermal treatment of strip, in particular strip of small thickness, of any type of steel, in particular carbon-manganese steels or carbon steels alloyed with nickel and/or chrome and/or molybdenum, non-oriented-grain silicon magnetic steels, and stainless steels, wound on coils when still at a high temperature, is characterized by the combination in a co-operation relationship, of the following steps: (i) winding of the strip at a temperature of between 600xc2x0 C. and the transformation temperature A3; (ii) transfer of the coils into an annealing furnace in a time of less than 30 minutes from winding, preferably less than 20 minutes, the furnace being heated to a temperature of between 560 and 870xc2x0 C. and maintaining the pre-selected temperature of steel for a pre-selected time; (iii) taking the coils out of the furnace at a temperature of less than 650xc2x0 C.
The temperature to which the furnace is to be heated depends upon the type of steel that is being treated and, in particular, in the case of stainless steels is between 650 and 850xc2x0 C., preferably between 800 and 850xc2x0 C.; for carbon steels it is between 570 and 760xc2x0 C., preferably between 670 and 730xc2x0 C.; for non-oriented-grain magnetic steels, it is between 660 and 830xc2x0 C., preferably between 670 and 710xc2x0 C.
Since according to the present invention it is possible to treat any type of steel, we shall now give the winding temperatures necessary for three important types of steel, i.e., carbon steels, non-oriented-grain magnetic steels, and stainless steels. For carbon steels, the coil winding temperature is between 600 and 770xc2x0 C., preferably between 700 and 750xc2x0 C.; for non-oriented-grain magnetic steels, the coil winding temperature is between 700 and 850xc2x0 C.; and for stainless steels, the coil winding temperature is between 650 and 850xc2x0 C.
In addition, according to the present invention it is possible to anneal the steel according to any one of the possible ways, and namely, passive annealing, in which the hot coil is charged into the furnace heated to a high temperature, the heat tranfer to the furnace after charging the coils being negligible or zero, so that the temperature of the furnace, and hence of the strip, slowly decreases in time; isothermal annealing, in which, after charging the coils into the fumace, the temperature of the furnace is kept at a desired level for a pre-set time, after which the temperature of the coils slowly decreases in time; and total annealing, in which after charging the coils into the furnace, the temperature of the furnace and hence of the coils is raised for a given period of time, until a pre-selected value is reached, after which the furnace and the coils are left to cool down slowly. In any case, the coils are taken out of the furnace at a given temperature, as will be seen later.
Consequently, in the case of passive annealing, the heating temperature of the furnace is between 600 and 860xc2x0 C., according to the type of steel, and the strip is kept at this temperature for less than 30 min, after which the furnace and strip are left to cool down for 8-28 hours, to obtain a maximum temperature of the strip, when it is taken out of the furnace, of less than 520xc2x0 C.
In the case of isothermal annealing, instead, the heating temperature of the furnace is between 580 and 830xc2x0 C., according to the type of steel, the coils being kept at this temperature for 4-15 hours, after which the furnace and strip are left to cool down for 4-16 hours, to obtain a maximum temperature of the strip, when it is taken out of the furnace, of less than 650xc2x0 C.
Finally, in the case of total annealing, the furnace is heated at a temperature of between 600 and 850xc2x0 C., according to the type of steel, the coils being kept at this temperature for 4-15 hours, after which the furnace and strip are left to cool down for 4-16 hours, to obtain a maximum temperature of the strip, when it is taken out of the furnace, of less than 650xc2x0 C.
It has moreover been found that the effectiveness of the process according to the present invention is improved if the coils are put into the furnace in a horizontal position. The improvement obtained is due, according to some experimental data, to the fact that, with the coils arranged in this way, circulation of the atmosphere inside the hole present around the axis of the coils is enhanced, so favouring a better uniformity of the thermal gradient along the radius of the coils themselves.